If you want to back up the VM's as if they are physical machines, then you buy 1 RAWS for each VM. However, if you want to backup the VM's as VM's, then you buy 1 Hyper-V licence for each VM host. With the Hyper-V licence, you are entitled to load RAWS on any number of VM hosted on the licenced host.

Note that you may have to also purchase an additional license if you wanted to backup up applications or DB's on that/those virtual boxe/s. IE Exchange, Sharepoint, SQL, domino, EV.. view the 2012 Licensing Guide below